Obesity is a major risk factor for many chronic illnesses, including diabetes and heart disease. With about 30 percent of U.S. adults now obese, treating these conditions is a leading driver of double-digit health care insurance premium hikes.

"These are very expensive patients," said Ken Thorpe, professor at Emory University's public health school and author of the study. "If insurers and employers are serious about reining in health care spending, then obesity prevention should be at the top of their agenda."

The Center for Consumer Freedom, an advocacy group financed by Coca-Cola, Wendy's and Tyson Foods, among others, has a Fourth of July message for you: worrying about the rapid rise in American obesity is unpatriotic.

"Far too few Americans," declares the center's Web site, "remember that the Founding Fathers, authors of modern liberty, greatly enjoyed their food and drink. ... Now it seems that food liberty - just one of the many important areas of personal choice fought for by the original American patriots - is constantly under attack." It sounds like a parody, but don't laugh. These people are blocking efforts to help America's children.

I've been looking into the issues surrounding obesity because it plays an important role in health care costs. According to a study recently published in the journal Health Affairs, the extra costs associated with caring for the obese rose from 2 percent of total private insurance spending in 1987 to 11.6 percent in 2002. The study didn't cover Medicare and Medicaid, but it's a good bet that obesity-related expenses are an important factor in the rising costs of taxpayer-financed programs, too. Fat is a fiscal issue.

But it's also, alas, a partisan issue.

First, let's talk about what isn't in dispute: around 1980, Americans started getting rapidly fatter.

Some pundits still dismiss American pudge as a benign "affliction of affluence," a sign that people can afford to eat tasty foods, drive cars and avoid hard physical labor. But all of that was already true by 1980, which is roughly when Americans really started losing the battle of the bulge.

The great majority of us (yes, me too) are now overweight, and the percentage of adults considered obese has doubled, to more than 30 percent. Most alarmingly, obesity, once rare among the young, has become common among adolescents, and even among children.

Is that a bad thing? Well, obesity clearly increases the risks of heart disease, diabetes, back problems and more. And the cost of treating these weight-related diseases is an important factor in rising health care spending.

So there is, understandably, a movement to do something about rising obesity, especially among the young. Bills that would require schools to serve healthier lunches, remove vending machines selling sweets and soda, and so on have been introduced in a number of state legislatures. By the way, Britain - with the second-highest obesity among advanced countries - has introduced stringent new guidelines on school meals.

But even these mild steps have run into fierce opposition from conservatives. Why?

In part, this is yet another red-blue cultural conflict. On average, people living outside metropolitan areas are heavier than urban or suburban residents, and people in the South and Midwest are heavier than those on the coasts. So it's all too easy for worries about America's weight to come off as cultural elitism.

More important, however, is the role of the food industry. The debate over obesity, it turns out, is a lot like the debate over global warming. In both cases, major companies protect their profits not only by lobbying against policies they don't like, but also by financing advocacy groups devoted to debunking research whose conclusions they don't like.

The pro-obesity forces - or, if you prefer, the anti-anti-obesity forces - make their case in part by claiming that America's weight gain does no harm. There was much glee on the right when a new study, using data from the Centers for Disease Control and Prevention, appeared to reject the conventional view that obesity has a large negative effect on life expectancy.

But as officials from the C.D.C. have pointed out, mortality isn't the only measure of health. There's no question that obesity plays an important role in many diseases that diminish the quality of life and, crucially, require expensive treatment.

The growing availability of such treatment probably explains why the strong relationship between obesity and mortality visible in data from the 1970's has weakened. But the cost of treating the obese is helping to break the back of our health care system.

So what can we do?

The first step is to recognize the industry-financed campaign against doing anything for the cynical exercise it is. Remember, nobody is proposing that adult Americans be prevented from eating whatever they want. The question is whether big companies will have a free hand in their efforts to get children into the habit of eating food that's bad for them.E-mail: krugman@nytimes.com